When my older son was in high school, he was competing in martial arts competitions. He became eligible to go to an important out-of-town event, and he was invited to go with the national team. How exciting was that?! He was sixteen. The national team men were older - some in their twenties - and, as polite as they were to me, I was quite aware that this group did not have a great reputation outside the competition arena.
I wrote the team captain a letter (that's how long ago this was) reminding him that they would be in charge of a high school boy and that I was sure they would take the responsibility seriously. My son was so mad you could have heard the explosion from here to North Carolina!
Months later, he "mentioned" that a couple of times the older guys were talking about doing certain things in their free time, and the captain would say, "No! We have ___ (my son) with us!"
So go ahead and call the boy's parents. Introduce yourself and tell them how much you like their son. They'll surely be glad to hear it. Let them know you're going out of town. Let your daughter (and son), as well as the parents, know that your house needs to be off limits to any and all company until you come back (yes, even at your children's advanced ages). That way it's just a house rule, without any suspicion cast upon the boy! Maybe this will be an opportunity for the boyfriend's parents to invite her to their house and get to know her.
Let your daughter also know that this is just part of your job, and that the boy's parents will not think any less of her for it (the boy won't either, once he gives it some thought).